This invention relates to a fuel assembly for fast reactors and more particularly to a fuel assembly for fast reactors in which stresses by interaction between a wrapper tube and a bundle of fuel elements can be reduced.
A conventional fuel assembly for fast reactors comprises a hexagonal wrapper tube and a bundle of fuel elements consisting of fuel pins and wire spacers wound around the fuel pins to prevent the fuel pins from contacting with each other and to keep a coolant passage therebetween. The fuel assembly is loaded in a fast reactor core and the fuel pins are burned for 2 to 3 years to produce heat energy. The heat energy is removed by a coolant (for example sodium). The maximum temperature of the coolant reaches to about 640.degree. to 680.degree.. Constructional materials of the fuel assembly, that is, fuel pin claddings, the wire spacers, etc. (these elements are made of stainless steel) are exposed to high speed neutron beams generated through nuclear fission and to a high temperature, as a result, they expand due to swelling and creep.
In the reactor core, the wrapper tube receives forces from inside of the wrapper tube toward the outside by pressure of the coolant, so that the wrapper tube creeps and expands. The fuel pins receive forces from the inside toward the outside by pressure of nuclear fission product gas contained in the fuel pins. As a result, creep takes place and the fuel pins expand. The wire spacers also expand due to swelling. Further, tensile strength caused by winding of the wire spacer causes creep in the wire spacers, whereby the wire spacers expand, but the volume of expansion is less than these of the fuel pins and the wrapper tube.
The fuel pins are bent complicatedly due to the difference of thermal expansion caused by uneven temperature distribution in coolant for the fuel assembly and due to an effect of so-called thermal expansion in addition to the above-mentioned swelling and creep, so that the fuel assembly presents complicated deformation.
At the beginning of loading the fast reactor with the fuel assemblies, a diameter (B) of the bundle of fuel elements in the wrapper tube is less than a dagonal length (E) of the inside of the wrapper tube. As the fuel assembly burns, both the diameter (B) and the diagonal length (E) become larger because of expansion of the various elements for fuel assembly. However, in general, since the expansion of the bundle diameter (B) is larger than that of the diagonal length (E) of the wrapper tube, the bundle diameter (B) reaches to the diagonal length (E), and finally the former becomes larger than the latter.
That the bundle diameter (B) of the fuel elements is larger than the diagonal length (E) means that the bundle of the fuel elements can not be contained within the wrapper tube geometrically. However, it is known that the bundle of the fuel elements can be contained really within the wrapper tube even if the bundle diameter (B) is larger than the diagonal length (E).
When the bundle diameter becomes larger than the diagonal length (E), the bundle of fuel elements receives such a force from the wrapper tube that the fuel pins are pressed strongly. In general, the expansion of the fuel pins progress faster than the expansion of the wrapper tube. Difference between the bundle diameter (B) and the diagonal length (E),(B-E), increases gradually in a burning period of the wrapper tube. When the difference turns to positive, the bundle of fuel elements receives stresses such that the fuel pins are pressed strongly. As the difference increases, the force that the fuel pins receive becomes. larger, and finally the fuel pins become damaged.
The conventional fuel assembly can not be allowed to stay in the core for a long time because it may be damaged by the interaction between the wrapper tube and the fuel elements. Japanese laid-open patent application No. 56-57985 discloses a proposal for preventing the damage caused by the interaction. In FIG. 5 of the Japanese laid-open patent application, a bundle of fuel elements are wound by a thick hollow tube of a diameter of about 4 mm. The Japanese laid-open patent application says that the hollow tube is crushed by the bundle of fuel elements as the fuel elements expand, so that the expansion is absorbed in the hollow tube. According to this method, a wrapper tube inherently becomes larger and consequently it is necessary to make the core larger. Further, the fuel assembly necessitates a long hollow tube because a winding passage on the periphery of the bundle of the fuel elements is large.